1. Field of the Invention
The present invention relates to a method for fabricating a thin film transistor liquid crystal display, and more particularly, to a method for fabricating a thin film transistor liquid crystal display, in which the conditions of plasma enhanced chemical vapor deposition are changed continuously so as to relieve stress in an insulating substrate and prevent the bending of the substrate.
2. Description of the Prior Art
Generally, the weight and thickness of a thin film transistor liquid crystal display, which is used in mobile communication devices such as notebooks, mobile phones and PDA, are being gradually reduced. For this purpose, the thickness and density of a glass substrate are being gradually reduced.
Thus, with a reduction in the substrate thickness, when multilayers (namely, an insulating layer of SiN, an active layer of amorphous silicon and an ohmic contact layer of impure amorphous silicon) which are formed to a large thickness at the highest temperature in a fabricating process of a thin film transistor are deposited by plasma enhanced chemical vapor deposition (PECVD), the phenomenon of bending of the glass substrate caused by stress remarkably appears.
In forming these prior multilayers, a SiN layer, an amorphous silicon layer and an impure ohmic contact layer are successively deposited to form an insulating layer, an active layer and an ohmic contact layer in a liquid crystal display device. Such multilayers are deposited under different deposition conditions (the kind of gas used, the flow rate of gas, the interval between electrodes, power and pressure).
Moreover, in the plasma enhanced chemical vapor deposition, since the transfer of energy by chemical reaction and plasma is achieved after the activation of reaction gas with plasma, most of compressive stress occurs in the insulating layer and the active layer.
In other words, the insulating layer is mainly made of SiN or SiON. A first insulating film of this insulting layer is deposited under conditions having little or no stress since it must have a thickness sufficient to provide the insulation between a gate electrode and a data electrode. On the other hand, a second insulating film forming an interface with the active layer is deposited under conditions having no roughness and excellent interface properties in view of the characteristics of a thin film transistor. Therefore such deposition conditions for the second insulating film are abruptly changed from those for the first insulating film.
Furthermore, even in the case of first and second amorphous silicon layers forming the active layer, the first amorphous silicon layer forming a channel layer at several hundreds or more of angstroms is deposited under a condition with less defect density and low deposition rate, but the second amorphous silicon layer is deposited under a condition of high deposition rate for the purpose of increasing productivity. This deposition condition for the second amorphous silicon layer is abruptly changed from one for the first amorphous silicon layer.
However, this abrupt change in deposition condition disadvantageously makes it difficult to match the insulating and active layers with the lower layer, so that stress in both the insulating substrate and thin film transistor is caused and as a result of this stress the TFT substrate undergoes bending. Thus, the characteristics of the thin film transistor deteriorate and the bending of the insulating substrate is increased so as to increase the possibility of the substrate breakdown in subsequent processes, thereby reducing yield.